The present invention may be more easily understood in the context of a camera that utilizes a color sensitive array of photodiodes to record an image. To provide color sensitivity, the photodiodes are typically divided into three classes of photodiodes that detect, respectively, red, green, and blue light. The various color sensitive photodiodes are dispersed over the array. For example, the detector array may consist of an array of pixels in which each pixel includes three photodiodes, one for measuring red light, one for measuring green light, and one for measuring blue light.
The color sensitive detectors are typically constructed by applying a pigment filter over a photodiode that is sensitive to light over a broad spectral range that includes red, blue, and green. For example, a color camera array can be fabricated by using conventional photolithography techniques to pattern either a red, blue, or green filter over each of the photodiodes in the array by selectively depositing the pigment in question. However, this process is limited by the materials that can be used for the pigment filter. Therefore, only limited color filter profiles can be created. For example, these filters are unable to block infrared (IR) light, and hence, such camera modules have to incorporate an additional IR blocking filter that significantly increases the costs of the camera.
In addition, the filter profiles obtained with the pigment filters do not match the standard filter profiles used to specify the color that will be perceived by a human observer at each pixel. Consider an application in which the color of a light source is to be reproduced on a printer for viewing by a human observer. While the light source may have a very complex spectrum, the eye perceives the source as having a single color that can be replicated by combining light from three colored sources. The printer is calibrated using some standardized color system such as the CIE 1931 standard. Given RGB values representing the intensity of light having the RGB spectral patterns in the standard system, the printer will produce the correct color. That is, a human observer will perceive the paper as having the same color as the light source even though the spectrum of light leaving the paper is different from that of the light.
The RGB values measured by the sensor using the pigment filters measure the intensity of light in a weighted wavelength band determined by the pigment filter transmission curve. Denote the measured intensities from the pigment filter light detectors by R′G′B′. In general, these R′G′B′ values differ from the RGB values that would be obtained by an ideal filter for the standard, since filter weighting functions are different. Hence, if these pigment-based values are sent to the printer, the printer will generate a color that is different from that of the light that was input to the color sensor.
Filters having more desirable color profiles can be fabricated by using interference techniques; however, these filters are difficult to construct over small area photodiodes. Hence, these filters are not useful for color cameras and the like in which very small pixel dimensions are needed. Interference filters are constructed by depositing multiple thin film layers of transparent dielectrics of different refractive indexes. The wavelength and filter profile are set by varying the thickness and index of refraction for the dielectrics. This provides great flexibility in the filter profile design. However, this technique is not suitable for CCD camera chips since it is difficult to pattern the individual pixels for high-resolution cameras. Hence, for a camera to utilize interference filters, three separate arrays on three separate chips are required. Each chip detects an image for light of one color. The three monochrome images would then be combined to provide the final color image. Since each chip requires only one type of filter, the problems associated with fabricating small individual photodiode-sized filters are eliminated. However, the need for three separate camera chips increases the cost and complexity of the camera optical system. In addition, the intensity of light available to each chip is reduced by a factor of three, which increases the amount of light needed to make a color measurement.